Modeling relaxation of a polymeric composite during loading and unloading

Polymeric composites exhibit highly nonlinear and rate dependent behavior during loading and unloading in off-axis directions. In this study, off-axis specimens were used to study this behavior in AS4/PEEK thermoplastic composite. The equilibrium states of stress during loading and unloading were determined by stress-relaxation and stress-recovery tests, respectively. It was found that the equilibrium stress-strain curves for loading and unloading are basically the same for the same loading-unloading rate. A one-parameter plasticity model was used to generate a master equilibrium plastic stress-strain curve for both loading and unloading. Stress relaxation curves and recovery curves were obtained during loading and unloading cycles. respectively, for various strain rates. Similarity between relaxation and recovery curves was noted, and the effective stress defined from the one-parameter plastic potential function was able to describe both curves.